Abstract
Photoprocesses in covalently linked bis(pyrochlorophyllide a) ethylene glycol diester molecules have been investigated by laser absorption and fluorescence techniques. Solvent interactions significantly alter the photochemical properties that resemble those of the special pair chlorophyll (P700) of photosystem I of green plants. Photoexcitation of the model system results in the formation of an S/sub 1/ state. This excited state returns to the ground state primarily by fluorescence and internal conversion. Under appropriate excitation conditions stimulated (laser) emission is observed. Long (30 ns) excitation pulses produce significant triplet populations. The configuration and possible reactions involving this triplet are discussed. In contrast, the folded pairs in methylene chloride/ethanol solution exhibit an unusually short fluorescence lifetime and a correspondingly low quantum yield. Some of the anomalous behavior of the folded pairs photoexcited in methylene chloride are shown to result from differences in ground-state structure and composition. Rapid internal conversion processes prevent the build-up of a significant excited-state population in this solvent system. The internal conversion process responsible for the lifetime shortening involves the formation of charge-transfer states. 53 references, 9 figures, 3 tables.
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